linux-kernel - Re: [PATCH] mm: pagevec: cleanup: drop pvec->cold argument in all places

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Message-Id: <20131003141008.0bf310445041a0919fe84fb9@linux-foundation.org>
Date:	Thu, 3 Oct 2013 14:10:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Bob Liu <lliubbo@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	viro@...iv.linux.org.uk, mgorman@...e.de, hannes@...xchg.org,
	riel@...hat.com, minchan@...nel.org, Bob Liu <bob.liu@...cle.com>
Subject: Re: [PATCH] mm: pagevec: cleanup: drop pvec->cold argument in all
 places

On Tue, 1 Oct 2013 22:47:36 -0700 (PDT) David Rientjes <rientjes@...gle.com> wrote:

> On Mon, 30 Sep 2013, Andrew Morton wrote:
> 
> > > Nobody uses the pvec->cold argument of pagevec and it's also unreasonable for
> > > pages in pagevec released as cold page, so drop the cold argument from pagevec.
> > 
> > Is it unreasonable?  I'd say it's unreasonable to assume that all pages
> > in all cases are likely to be cache-hot.  Example: what if the pages
> > are being truncated and were found to be on the inactive LRU,
> > unreferenced?
> > 
> > A useful exercise would be to go through all those pagevec_init() sites
> > and convince ourselves that the decision at each place was the correct
> > one.
> > 
> 
> Agreed, and the "cold" argument to release_pages() becomes a no-op if this 
> patch is merged meaning that anything released through it will 
> automatically go to the start of the pcp lists.  If the pages aren't hot 
> then this is exactly the opposite of what we wanted to do; the fact that 
> the pvec length doesn't take into account the size of cpu cache can almost 
> guarantee that everything isn't cache hot.

The hot/cold pages code was very marginal when we first merged it and I
suspect it has rotted since.

It would be a useful exercise for someone to disable it then run some
benchmarks with a view to removing it all.  But the problem I have with
this approach is perhaps the code *could* become effective if some
careful maintenance work was done on it - we should at least get the
hot/cold decisions optimised before making a decision about the overall
desirability of keeping it.



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